Can allocating more RAM to Minecraft help with error 666, and how do I do it?

Allocating more RAM *might* help if the error is related to insufficient memory, but it's not a guaranteed fix. In the Minecraft Launcher, go to 'Installations', select your profile, click 'Edit', then 'More Options'. In the 'JVM Arguments' field, change '-Xmx2G' (or similar) to a higher value like '-Xmx4G' (for 4GB of RAM), but don't allocate more than half of your system's RAM.